Much attention is being given to silicon photonics technology that integrates optical devices and electronic devices at high density in a unified manner on the same silicon substrate. There are great expectations for this technology in applications not only in the field of optical communications, but in optically interconnect of integrated circuits, and R&D is progressing vigorously at present.
In silicon photonics technology, it is common to use a silicon on insulator (SOI) substrate, in which single crystalline silicon (c-Si) is formed on top of a buried oxide (BOX) film, but there is a problem that an extremely high temperature process of 1100° C. or higher is required to form the c-Si, and the SOI substrate is expensive.
Also proposed is a silicon photonics technique that uses hydrogenated amorphous silicon (a-Si:H) films, which can deposited at low temperatures; i.e., 400° C. or lower, while exhibiting performance of a nature that rivals c-Si or surpasses the same in some characteristics including nonlinear optical characteristics, and R&D of a diverse range of passive devices has been conducted to date.
In fact, while it goes without saying that active devices including high-speed electro-optical conversion devices or optical switching devices of a nature that switch optical paths by electrical signals are required to perform communications of electrical signals with light waves as carrier waves, there are not many reports related to these. One reason is that a-Si:H has low electrical characteristics of mobility, conductivity, or the like because of its non-crystalline material.
Meanwhile, as is disclosed in non-patent document 1, it is known that with carriers in a-Si:H internally injected or excited with some kind of process relaxes in a very short time, typically in sub-picoseconds, and this is due to the fact that relaxation of wave function of the carriers from an extended state to a localized state, specifically a tail state, is extremely fast. Since the tail state stem from variation of Si bond lengths or bond angles the high-speed carrier relaxation is unique phenomenon due to a random structure.
Many electro-optical modulators based on c-Si have been reported on (for example, patent document 1), but in the case of c-Si, one of major factors is that the carrier relaxation time is comparatively slow and the modulation speed is limited. In short, it can be said that in terms of carrier relaxation time, a-Si:H has more advantageous characteristics as a high-speed modulator compared with c-Si.
FIG. 11 illustrates one example of an a-Si:H based electro-optical modulator fabricated with a low-temperature process using a-Si:H described in non-patent document 2.
This electro-optical modulator comprises: an i-type a-Si:H layer 103 not doped with impurities as a waveguide core on a silicon substrate 101; a p-type a-SiC:H layer 102 as a lower cladding, doped with impurities as a p-type semiconductor in hydrogenated amorphous silicon carbide (a-SiC:H), which has a somewhat lower refractive index than i-type a-Si:H while likewise being capable of low-temperature growth in the same manner as i-Si:H; an n-type a-SiC:H layer 104 doped with impurities as an upper cladding layer on the i-type a-Si:H layer 103; and a zinc oxide/aluminum electrode 105 on the top.
The structure illustrated in FIG. 11 constitutes an optical waveguide structure having the i-type a-Si:H layer 103 with the highest refractive index as the waveguide core, and at the same time the p-type layer (102), i-type layer (103), and n-type layer (104) constitutes a pin structure.
In this electro-optical modulator, the conductivities of the a-SiC:H of the n-type layer (104) and the p-type layer (102) are 2.3×10−6 S/cm and 1.9×10−8 S/cm, respectively.
The abovementioned electro-optical modulator is connected to an external power source so that voltage is applied to the i-type a-Si:H layer 103 through the silicon substrate 101 and the zinc oxide/aluminum layer 105 on top of the waveguide. When reverse bias is applied to the i-type layer (103), depletion layers spread to sides of each of the p-type layer (102) and the n-type layer (104), the carrier density of the i-type layer (103) decreases, and the refractive index of the i-type a-Si:H layer 103 increases.
The phases of the light waves propagating inside the waveguide with the i-type a-Si:H layer 103 as the waveguide core can be modulated thereby, and in this case the operating speed of the abovementioned electro-optical modulator is limited mainly in accordance with the mobilities and conductivities of the p-type layer (102) and the n-type layer (104). However, in the case of the electro-optical modulator described in non-patent document 2, it is extremely difficult to obtain a high-speed modulation operation of a nature that exceeds 1 Gbps, because a-SiC:H, which has extremely low mobility and conductivity, is used in the p-type layer (102) and the n-type layer (104). Specifically, it cannot be said that the high-speed carrier relaxation characteristics of a-Si:H are exploited in this electro-optical modulator.